Unidentifiable popping sound

/\.   What Fuzz said!

Have you isolated all of the tube equipment to determine if anything else that has tubes in it might be the culprit? I had a very similar experience with some NOS tubes in my preamp years ago. Yes, they were the correct values and exact same type for this preamp. After I turned on the equipment and a short while later, I would hear a pop. I thought something was going to blow up the system. I started with the preamp and replaced the NOS tubes with more modern tubes, PSVayne, and voila, no more pops. Luckily,, this was the first step in my investigation which isolated the issue. I examined the NOS tubes, RFT Germany, and noticed that perhaps a filament was more than likely the cause. It was hanging on barely by a thread, probably arcing or something. Not sure, but when I replaced them, everything was back in order and never had the problem since. Popping noise is never good when it comes to Electonics. Also, make sure that when you are switching between components, that all connections are properly installed. If you have WBT or similar connectors which require an extra turn or two to lock the connector, you may also experience some type of weird anomalies such as popping or hum…I have experienced this as well. 

@fuzztone good call. will try that.
@audioquest4life Nagra is very, very good at QC and it happened with both the tubes that came with the Jazz (picked it up pre-owned), and with new ones I got directly from Nagra. If it is the preamp, I'm not sure it's the tubes (but anything is possible at this point!).

So, you hear popping 10 minutes AFTER you power the system down? Is this popping through the speakers? Not likely.